Spiral platers are used to deposit microbe-containing liquid samples in a spiral pattern onto the surface of agar media contained in petri dishes. An essential part of such devices is a hollow stylus connected to a hollow plunger syringe, with an on-off valve mounted on top of the hollow plunger, further connected by tubing to a vacuum flask. With the valve open, the vacuum will draw liquid from the container into which the tip of the stylus is immersed, to fill the stylus tubing as well as the syringe. Simultaneous displacement of the syringe plunger, radial motion of the stylus tip riding on the surface of the agar, and rotation of the turntable holding the petri dish, deposit the microbe-containing sample in a spiral pattern and in amounts determined by the controlled rate of displacement of the syringe plunger. Earlier instrumentation of spiral platers utilize cam activation of plunger motion and manually controlled valve operation as well as manual movement of the stylus to and into sample and cleaning fluid containers. More recent spiral plater designs utilize microprocessor control of (1) plunger motion, (2) valve on-off activation and (3) automated motion of the stylus to a number of cleaning fluid containers, a sample holding container and immersion of the stylus into these containers. Such a microprocessor controlled spiral plater is sold by Spiral Biotech, Inc. of Bethesda, Md., as the AUTOPLATE Model 3000.
Practical utilization of the spiral plating method requires that (1) the stylus tubing holding the sample to be deposited is clear of any microbes from the previous sample and (2) that disinfectant used to clean the stylus prior to filling with the next sample is not carried over into this sample. The former would cause overestimation of the density of microbes in the sample under test while the latter would produce an underestimate by killing some or all of the microbes in the sample under test. The method used to avoid the above errors consists of opening the vacuum control valve and dipping the stylus a few times into and out of a container with disinfectant, such as a solution of sodium hypochlorite, followed by dipping the stylus a few times into and out of a cleaning solution, such as sterile water. The principal function of immersion into the disinfectant is to kill any microbes on the outer surface of the stylus tip. Multiple immersions into disinfectant and cleaning solutions serve to cause rapid, and sequential, flow of liquid and air--when the stylus is out of the liquid container--thereby scrubbing the inside of the stylus tubing to remove material from the previous sample.
Upon completion of the cleaning operation, the stylus tubing and syringe are allowed to fill with sterile water, preceding the intake of the next sample. In the earlier cam activated spiral platers, sample intake is also done by manually opening and then closing the vacuum valve; in the microprocessor controlled spiral plater, it is done automatically by activation of syringe plunger motion, with the vacuum valve closed.
Frequent manual operation of replenishing disinfectant and cleaning liquids is not consistent with the automated operation of microprocessor controlled spiral platers. Therefore, a method that cleans effectively and requires only occasional replenishing is highly desirable.
Liquid levels are important since, for example, allowing the liquid level to go below the immersion level of the stylus in a preceding container will likely cause carry over of microbes from the earlier sample and/or carry over of disinfectant into the next sample. In the AUTOPLATE Model 3000 this design requirement is addressed by seeking to continuously and automatically maintain a constant liquid level during operation. Specifically, a gravity syphon system is used in which liquid from an inverted container into an attached cup with upright openings is limited by atmospheric pressure on the free surface of the liquid. A vent tube is provided for the inverted container so that when liquid level in the associated open cup is lowered due to suction by the immersed stylus, air can pass into the inverted container reducing the partial vacuum in the container, thus allowing flow into the cup until the vent is again submerged below the liquid level in the cup. The liquid containing bottles, pressure equalization ports and associated manifolds to the stylus immersion openings add substantially to the cost of the instrument, since they must operate reliably--without leakage or significant change in liquid level--following repeated autoclaving to sterilize the assembly in the course of normal utilization, and because materials must be resistant to the corrosive effect of disinfectants such as sodium hypochlorite.